The invention relates to the field of amplifiers.
More precisely, the invention relates to a method and a system for linearizing an amplifier.
The invention is particularly, but not exclusively, applicable to linearizing amplifiers used in cellular radiocommunications systems (such as, in particular, the Global System for Mobile communications 900 MHz (GSM 900) (a public radiccommunications system operating in the 900 MHz band), the Digital Cellular System 1800 MHz (DCS 1800), the Personal Communication System (PCS), IS95, the Universal Mobile Telephone System (UMTS), the International Mobile Telecommunications 2000 system (IMT-2000), CDMA-One, etc.
A mobile terminal generally includes an amplifier in its transmission system.
Frequently, it is desirable to obtain linear amplification. Since an amplifier is never exactly linear, it is necessary to linearize the amplifier.
In radiocommunications systems, the non-linearity of the amplifier in the radio transmission system is detrimental because it distorts the transmitted signal, thereby degrading the spectrum of the signal, and thus causing interference signals to be generated in channels adjacent to the working channel. In other words, the non-linearity of the amplifier causes an increase in the level of transmission interference in the adjacent channels. This gives rise to failure to comply with specifications aiming to limit interference. Such failure to comply with the specifications is further accentuated when modulation having a non-constant envelope (which is more sensitive to non-linearity defects) is used upstream from the amplifier. It can thus be understood that it is necessary to linearize the amplifier.
Several solutions for performing such linearization are known from the state of the art.
A first known linearization solution consists in using the amplifier well within its capacities, so as to operate in a linear zone. The lower the power level, the smaller the non-linearity manifested by the amplifier. That first known solution suffers from the major drawback of degrading amplifier efficiency. Thus, if such an amplifier is included in a mobile terminal, it reduces battery life between charges.
A second known linearization solution consists in using an analog feedback correction loop (cf FIG. 1). Unfortunately, the correction loop requires additional analog components which firstly suffer from defects affecting the performance of the resulting overall system, and secondly are subjected to drift over time, causing the effectiveness of the correction to vary.
A third known linearization solution consists in using an analog feedforward correction loop (cf FIG. 2). The drawback with such a correction loop is that it requires a second amplifier which must be very linear.
A fourth known linearization solution is described in Patent Document EP 522 706 A2. That document describes a linearization method comprising a prior testing stage and an amplification stage. During the prior testing stage, the following are performed successively: a test signal is amplified; the corresponding output signal is sampled at a low rate over a long period; the Fourier transform is applied to the samples of the amplified test signal; the amplitudes of the harmonics are computed so as to reconstruct the output signal; the input signal is compared with the reconstructed output signal of the amplifier, so as to determine the distortion parameters specific to the amplifier, and a table is constructed of predistortion parameters which are such that they cancel the distortion parameters specific to the amplifier. During the amplification stage, the predistortion parameters are applied to the signal to be amplified, so that the amplified signal at the output of the amplifier is not distorted.
In other words, in the fourth solution, the principle of predistortion is used which consists in compensating for the non-linearity of the amplifier by adding a unit of inverse non-linearity at the input of the amplifier, so that the juxtaposition of the two non-linear units has a linear transfer function. The test stage which, in the fourth solution, consists in comparing the input test signal with the corresponding output signal, makes it possible to determine the distortion (or non-linearity) due to the amplifier.
The fourth known solution is not advantageous because it requires many samples to be taken at the output of the amplifier, and therefore requires a large amount of computation, considerable memory space, and a long time for acquiring (i.e. recovering) and processing the samples at the output of the amplifier. In addition, the results obtained are not accurate. It should be noted that a long acquisition time can be incompatible with certain operational constraints allowing only a small amount of time for acquisition: signals that are not steady, or time limits for transmission of test signals).
To sum up, it appears that all of the above-mentioned known solutions which aim to enable an amplifier to be linearized suffer from drawbacks, namely, in particular, their high cost considering the performance obtained.
A particular object of the present invention is to mitigate the various drawbacks of the state of the art.
More precisely, one of the objects of the present invention is to provide a method and a system of digitally linearizing an amplifier, the method and system requiring lower computation power than the above-mentioned known solutions.
An additional object if to provide such a method and such a system that require only a small amount of memory space.
Another object of the invention is to provide such a method and such a system that make it possible to reduce the time required to acquire the samples at the output of the amplifier, as compared with the solutions known from the state of the art.
Another object of the invention is to provide such a method and such a system that make it possible to reduce the time required to process the output samples, as compared with the solutions known from the state of the art.
A further object of the invention is to provide such a method and such a system that make it possible to obtain results that are very accurate.
Another object of the invention is to provide such a method and such a system that are low in cost.
These various objects and others that appear below are achieved by the invention by means of a method of digitally linearizing an amplifier having non-linearity, the method being of the type comprising:
a prior determination stage in which correction parameters for correcting said non-linearity are determined, as a function of an amplified test signal present at the output of the amplifier when a test signal is applied to the input thereof; and
a linear amplification stage comprising the following steps:
causing an input signal which is to be amplified by said amplifier to be predistorted as a function of said correction parameters so as to obtain a predistorted signal; and
causing said predistorted signal to be amplified by said amplifier so that the amplified predistorted signal present at the output of said amplifier corresponds to said input signal as amplified linearly;
xe2x80x83wherein said determination stage in which the correction parameters are determined comprises the following steps:
acquiring said amplified test signal in the form of samples in baseband;
defining a modelled amplifier, by computing parameters of a predetermined amplifier model on the basis of said samples of the amplified test signal; and
computing said correction parameters on the basis of said modelled amplifier so that they make it possible to linearize said modelled amplifier.
The present invention thus provides a novel way of determining the predistortion relationship (i.e. the correction parameters) to be applied upstream from the amplifier in order, ultimately, to obtain amplification that is linear.
More precisely, the general principle of the invention consists in modelling the real amplifier, and then in computing the predistortion relationship on the basis of the modelled amplifier (and not on the basis of the real amplifier) in order to compensate for its non-linearity.
In other words, the work is done on a model of the amplifier and not, as in above-mentioned Patent Document EP 522 706 A2, merely on a comparison between the input test signal and the corresponding output signal (i.e. the amplified test signal). It is important to note that the fact that a modelled amplifier is used makes it possible to obtain improved accuracy on the correction parameters to be applied to the input signal, and thus improved linearization of the amplifier. In general, the closer the modelled amplifier is to the real amplifier, the better the resulting linearization.
It is recalled that, when applied to a mobile terminal, the linearization method of the invention makes it possible to limit interference between channels. Clearly, however, the method of the invention is not limited to being applied to the case when the amplifier is incorporated in a mobile terminal, but rather it can be considered regardless of the use of the amplifier.
Advantageously, said step in which the modelled amplifier is defined comprises the following steps:
performing spectrum analysis on said amplified test signals to estimate the complex amplitudes of a predetermined number of harmonics at known frequencies, said harmonics being generated by said non-linearity of the amplifier; and
computing the parameters of the predetermined amplifier model by identifying firstly the estimated complex amplitudes and secondly mathematical expressions corresponding to complex amplitudes of harmonics at said known frequencies for a signal present at the output of the modelled amplifier, said mathematical expressions being computed on the basis of said predetermined amplifier model, by assuming that the test signal is applied to the input thereof.
Estimating the complex amplitudes of certain cross-modulation harmonics makes it possible to characterize the amplitude-amplitude (AM-AM) and amplitude-phase (AM-PM) non-linearities of the amplifier.
In addition, since a limit is set at a predetermined number of harmonics whose frequencies are known, the time taken to acquire the samples of the amplified test signal can be short.
Preferably, said spectrum analysis performed on the samples of the amplified test signal belongs to the group comprising:
parametric spectrum analyses; and
spectrum analyses by sampling followed by a Fourier transform.
It should be noted that parametric spectrum analysis offers the advantage of requiring fewer calculations, and less memory space, and of requiring a shorter sample input time. In addition, it is possible to simplify this type of analysis considerably insofar as firstly the waveform of the signal is known (superposition of a plurality of sinewaves), and secondly the frequencies of the spectrum lines (or harmonics) whose amplitudes are to be determined are known.
In a preferred implementation of the invention, said step in which the complex amplitudes of harmonics are estimated consists in computing:
H{tilde over ( )}=M S
where
H{tilde over ( )}=[H1{tilde over ( )} H2{tilde over ( )} . . . Hp{tilde over ( )}]T is the vector of the complex amplitudes of p harmonics in question;
S=[s0 s1 . . . sN]T is the vector of N samples of the amplified test signal; and
M is a predetermined matrix (p,N).
In this way, the vector H{tilde over ( )} can be computed very rapidly. The matrix M, which depends only on the frequencies of the harmonics that are to be characterized, is entirely known, and can be computed once and for all and stored in a memory. N is also referred to as the xe2x80x9csampling windowxe2x80x9d.
Advantageously, said matrix M can be written:
M=([ZHZ]xe2x88x921ZH)
where
ZH is the Hermitian transpose of Z;
zi=exp(2jxcfx80fi), fi being the frequency of the ith harmonic in question; and   Z  =      [                            1                          1                          ⋯                          1                                                  z            1                                                z            2                                    ⋯                                      z            p                                                ⋮                          ⋮                          ⋯                          ⋮                                                  z            1                          N              -              1                                                            z            2                          N              -              1                                                ⋯                                      z            p                          N              -              1                                            ]  
In a preferred implementation of the invention, said predetermined amplifier model is a polynomial model.
Clearly, the higher the degree of the polynomial that models the amplifier, the greater the accuracy of the modelled amplifier. The transfer function modelling the amplifier (voltage/voltage transfer function) may be given in RF by a polynomial, it nevertheless being possible to determine the complex coefficients of such a polynomial in baseband.
Preferably, said polynomial model is of odd order and does not take account of the even-order terms.
It is possible to take no account of even-order terms since they do not generate harmonics in the band of the high-frequency signal.
Preferably, said polynomial may be written y{tilde over ( )}=x{tilde over ( )}*G{tilde over ( )}a in baseband, where:
the input may be written x{tilde over ( )}=xi+j*xq 
the output may be written y{tilde over ( )}=yi+j*yq 
Ga{tilde over ( )}(Px)=a1{tilde over ( )}+(3/4)xc2x7a3{tilde over ( )}xc2x7(Px)+(5/8)xc2x7a5{tilde over ( )}xc2x7(Px)2+(35/64)xc2x7a7{tilde over ( )}xc2x7(Px)3+ . . .
xe2x80x83where:
Px=xi2+xq2 
a1{tilde over ( )}=aixc2x7exe2x88x92jxcfx89xcfx84i
xi and xq are respectively the in-phase component and the quadrature component of x;
yi and yq are respectively the in-phase component and the quadrature component of y; and
the coefficients ai and the associated delays xcfx84i are parameters of said predetermined amplifier model.
Going over to baseband makes it possible to facilitate computation of the parameters characterizing the modelled amplifier.
Preferably, said test signal is a signal having n tones whose frequencies and amplitudes are known.
In this way, it is possible to determine, a priori, the frequencies at which the cross-modulation products can appear in the amplified test signal. By means of this information, it is possible to use amplifier model identity algorithms that are much simpler and of higher performance than those generally used.
Preferably, the test signal is made up of two sinewaves (n=2) having the same amplitude.
The invention also provides a system for digitally linearizing an amplifier having non-linearity, in which system the means for determining correction parameters comprise:
means for acquiring said amplified test signal in the form of samples in baseband;
means for defining a modelled amplifier, by computing parameters of a predetermined amplifier model on the basis of said samples of the amplified test signal; and
means for computing said correction parameters on the basis of said modelled amplifier so that they make it possible to linearize said modelled amplifier.